Why is my render taking so long?
Well, there's isn't really a magic button you can press to make your renders finish and clean up faster, but here are a few points that may help, or may not.

Hardware

The main thing that comes to mind is your hardware. If you don't have somewhat nice hardware, you aren't going to get very far.

For the built in cycles renderer, it is good to have a NVidia GPU with CUDA (if you have a nvidia card, learn to setup cuda here). This is much faster than the normal CPU rendering. Having a decent CPU is also good if you don't have a GPU

Scene

Another factor, is going to be your scene. It can really be a lot of things. 

Since you said you where doing an interior, that fact that the light will be bouncing around more will slow down your render. If you have complex materials, naturally, it will slow down your render. If you have lots of lights, that can do it also.

There's really lots of stuff it can be. Without knowing more about your system and scene, it's really hard to say.

Render Settings

Another big factor to consider are your render settings. Number of light bounces, number of samples, and resolution. High light samples are commonly seen as being the source of lengthy render times, but resolution is another significant factor. For a scene I was rendering at 1920x1080, it was taking around 5 minutes per frame. I reduced the resolution to 1280x720, and it reduced the time it took to render each frame to 30 seconds. Consider reducing your resolution if where you share/use the file doesn't use or need the extra pixels. 


This article should give you a few extra pointers on what you can do to speed up cycles.

And for Blender Internal, this article could help out a bit also.